The present invention relates to resins useful in curable formulations, particularly for coating applications.
Coating formulations that contain curable epoxy resins, isocyanates, and other resins and their curing agents are commonly used to put decorative and/or protective coatings on indoor and outdoor articles. These formulations are usually applied as powders or as solutions of curable resin in aqueous or organic solvents. Solutions are commonly applied by spraying, brushing, dipping, rolling and other methods. Powders are commonly applied by fluidized bed or electrostatic spraying of powder onto a heated substrate which causes the powder to melt, flow, consolidate and cure.
Viscosity (or melt viscosity in the case of powder coating formulations) is a critical characteristic in solvent-borne and powder-coating formulations. The viscosity is ideally low enough so that the formulation flows and spreads evenly before it cures, in order to provide a smooth, even appearance and a surface which is free of holes and other flaws. High viscosity also makes it more difficult to obtain adequate wetting using the formulation. Inadequate wetting may reduce coating adhesion or hurt other coating properties.
Viscosity control is becoming even more important because of current attempts to limit emissions of volatile organic compounds (VOC). In order to reduce their use of VOC, coaters either use formulations which contain higher solid concentrations and lower solvent concentrations than previously used, or substitute powder coatings into applications where they were not used previously because of uneven appearance. In both instances, the higher viscosity of the formulation or melt makes it more difficult to apply a smooth coating with even appearance.
The viscosity of many formulations decreases as the formulation is heated for curing, providing an opportunity for the formulation to flow and cover the surface. However, the formulation builds molecular weight rapidly as it cures, so that the viscosity quickly rises again. The coating formulation would have more opportunity to spread evenly before it cures if the buildup of viscosity were slowed.
Several methods are known to slow the reaction of curable resin and curing agent. For instance, the formulation may contain less reactive reagents or latent catalysts. Those methods control the buildup of viscosity, but also lengthen the time needed to complete the curing reaction. This is economically undesirable, because the cure requires longer residence time and more energy in curing ovens.
What is needed is a relatively low viscosity curable composition which builds viscosity in a slower, more controlled fashion, but completes the curing cycle in about the same total time.